The present invention relates to apparatus for the pretreatment of sea water or briny water to prevent scaling in desalination apparatus.
It is known that apparatus for producing fresh water from sea water or briny water which operates on the evaporation-distillation principle has a limited capacity owing to upper temperature and operating limits imposed by scaling.
The principal ions directly or indirectly responsible for scaling are, in the case of sea water, calcium and magnesium cations and bicarbonate and sulfate anions; the nature of the deposit which forms is dependent on the operating temperature.
To prevent calcium carbonate deposits, it is possible to use adsorbents or growth inhibitors such as polyphophates. It is also possible to decompose the carbonate by the injection of an acid, for example, sulfuric acid, and degassing. In the case of calcium sulfate, a conventional method of preventing crystallization consists in limiting the temperatures and concentrations of the brine so that the thermodynamic conditions of precipitation never occur in the course of the evaporation-distillation cycles. However, this method, which is advantageous because it does not necessitate any initial chemical processing, is obviously subject to a certain temperature limitation, i.e. on the order of 120.degree.C, and, therefore, is not compatible with the objective of obtaining maximum yields from the evaporation-distillation process. It is also possible to remove the greater part of the calcium ions by a softening process, e.g. by ion exchange, although the cost of such a process is prohibitive.
It is known that the scaling problem can be economically reduced to permit operation of the evaporators at high temperatures in excess of 120.degree.C by the introduction of crystalline nuclei into the heated sea water. The addition of the nuclei to a metastable or over-saturated solution results in the deposition or crystallization of the salts in solution on the nuclei, and thereby avoids nucleation of these salts on the heated walls of the evaporator.
In this type of treatment, crystalline nuclei (for example, anhydrite) are added to the untreated sea water before it enters the evaporation unit wherein it is progressively heated by its passage through the condensers of the unit. This preheating of the crude sea water, a conventional step in any distillation process, is continued to a higher temperature than that of the first stage of the evaporation unit and to a higher temperature than that corresponding to the limit of solubility in water of the alkaline-earth carbonates and calcium sulfate scaling agents.
In the course of this heating operation, crystallization of these compounds occurs and the carbonates and sulfate crystals grow on the suspended anhydrite nuclei.
Sea water at a high temperature, e.g. 170.degree.C, containing a mixture of crystals in suspension is thus obtained. Before introduction into the first stage of the evaporator, it is necessary to clarify this water by removing the suspended crystals. If the suspended crystals are not removed they will redissolve during the cooling of the water in the successive stages of the evaporation unit and form scale on the various ducts in the evaporation unit. This scaling would result in lowering the efficiency of the desalination process. Thus, it is absolutely essential to remove the crystals of alkaline-earth carbonate salts and calcium sulfate before cooling and/or evaporation.
The temperature and pressure of the water requiring clarification (temperature in excess of 120.degree.C) make it difficult and costly to employ a standard decanter/centrifuge or filter; such apparatus must be specifically designed to permit reliable operation under these conditions.